This invention relates to tool steel alloys, and in particular, to a high speed tool steel alloy and a powder metallurgy article made therefrom that has a unique combination of hardness and toughness.
AISI Type T15 alloy is a known tungsten high speed steel alloy. The Type T15 alloy is considered to be among the premium high speed tool steel grades because it has a combination of hardness and wear resistance that is superior to other high speed tool steel alloys such as Types M2 and M4. Type T15 alloy provides a hardness of about 66 to 67 HRC at room temperature. A higher carbon version of Type T15 alloy that is capable of providing a room temperature hardness of 67 to 68 HRC has been sold in the U.S. However, a demand has arisen in the tooling industry for a high speed tool steel alloy that provides greater combined levels of hardness, including elevated temperature hardness, and wear resistance than the known grades of high speed steel alloys, such as Type T15.
Currently there are essentially two types of materials that are available for the more demanding tooling applications such as metal-cutting tools and gear hobs: conventional high speed tool steels and cemented carbide materials. The known high speed steel alloys, even when produced by powder metallurgy techniques, leave something to be desired for extended tooling runs because tools manufactured from those materials lack sufficient wear resistance, room temperature hardness, and hot hardness. There is presently a trend in industry toward use of dry machining as opposed to the use of cutting fluids because of the potential environmental hazard associated with conventional cutting fluids. Metal cutting tools are likely to be subjected to significantly higher operating temperatures when used in dry machining operations. Most of the known high speed steel alloys are not suitable for use in dry cutting operations because their wear resistance and hardness degrades very rapidly under the extreme temperature conditions.
To avoid the limitations of the known high speed tool steels, one approach has been to produce cutting tools with a very hard surface coating to improve the service life of these cutting tools. Such a coating is typically applied by either physical vapor deposition (PVD) or chemical vapor deposition (CVD). Such coatings are typically harder than about HRC 70, which is much harder than the base tool steel. It would be advantageous to provide a tool steel alloy having increased hardness to back up the very high hardness coating.
Because of the disadvantages associated with the known high speed steel alloys as outlined above, cemented carbide materials have become very attractive for making cutting tools. Cemented carbide materials provide very high hardness, both at room and elevated temperatures, and very good wear resistance. Although cemented carbide tooling materials provide excellent hardness and wear resistance, they have several disadvantages. For example, carbide tooling is very expensive to produce, not only because of the cost of making the carbide blanks, but also because of the extra cost of forming the cutting tools from those blanks. In addition, carbide tools have very low toughness and special care must be taken to prevent fracture during service. Also, extremely rigid machines must be used with carbide tooling, and therefore, a large portion of existing cutting machines cannot be safely run with carbide tooling.
The alloy according to the present invention, and a consolidated powder metallurgy article formed therefrom, resolve to a large degree several of the problems associated with the known high speed tool steels and cemented carbide materials. In general, the invention provides a high hardness, high speed tool steel alloy having a unique combination of hardness, hot hardness, and toughness. The broad, intermediate, and preferred weight percent compositions of the alloy according to this invention are set forth in Table 1 below.
The balance of the alloy is essentially iron and the usual impurities found in commercial grades of high speed tool steels intended for similar types of service. The carbon content of the alloy according to this invention is controlled such that the parameter xcex94C is about xe2x88x920.05 to xe2x88x920.42, better yet about xe2x88x920.10 to xe2x88x920.35, and preferably about xe2x88x920.15 to xe2x88x920.25. xcex94C is calculated as follows.
xcex94C=((0.033W)+(0.063Mo)+(0.06Cr)+(0.2V))xe2x88x92C
where ((0.033W)+(0.063Mo)+(0.06Cr)+(0.2V)) is the carbon balance of the alloy, C is the actual carbon content of the alloy, and W, Mo, Cr, V, and C are given in weight percent.
Here and throughout this application, the term xe2x80x9cpercentxe2x80x9d or the symbol xe2x80x9c%xe2x80x9d means percent by weight unless otherwise indicated.
At least about 1.85% carbon is present in this alloy to benefit the high hardness provided by the alloy in the hardened and tempered condition. Carbon combines with the carbide-forming elements in this alloy to produce carbides that contribute to the excellent wear-resistance provided by the alloy. The alloy preferably contains at least about 1.90% carbon. Too much carbon adversely affects the toughness provided by this alloy, and at very high levels, can adversely affect the attainable hardness of the alloy. Therefore, carbon is restricted to not more than about 2.30% and preferably to not more than about 2.20% in this alloy. Because carbon is depleted when carbides are formed in the alloy, the amount of carbon is controlled so that there is sufficient carbon to permit the attainment of the desired hardness provided by the alloy as well as to permit the formation of an adequate volume of hard carbide particles to provide the desired wear resistance. To that end we use the factor xcex94C described above whereby the amount of carbon present in the alloy can be controlled to provide the unique combination of properties that are characteristic of this alloy.
This alloy contains at least about 0.15% manganese to benefit the hardenability of the alloy. In the resulfurized embodiment of the alloy according to this invention, manganese combines with sulfur to form manganese-rich sulfides that are highly beneficial to the machinability of the alloy. Too much manganese causes brittleness in this alloy. Therefore, manganese is limited to not more than about 1.0% and preferably to not more than about 0.90%.
At least about 0.15%, better yet at least about 0.50%, and preferably at least about 0.55% silicon is present in this alloy to benefit the hardenability of the alloy and its hardness response. Silicon also contributes to the fluidity of the alloy in the molten state which facilitates the atomization of the alloy for powder metallurgy applications. Too much silicon adversely affects the good toughness provided by this alloy. Therefore, the amount of silicon is restricted to not more than about 1.0%, better yet to not more than about 0.80%, preferably to not more than about 0.75%.
This alloy may contain up to about 0.30% sulfur to form manganese-rich sulfides which benefit the machinability of the alloy as described above. At least about 0.06% sulfur has been found to effective for that purpose. In order to form a sufficient quantity of sulfides to benefit the machinability property, the amounts of manganese and sulfur present in the alloy are selected to provide a Mn-to-S ratio (Mn:S) of about 2:1 to 4:1, and preferably about 2.5:1 to 3.5:1. Sulfur adversely affects the toughness provided by this alloy and, therefore, it is restricted to not more than about 0.30% in the enhanced machinability embodiments of this alloy. Where enhanced machinability is not needed, sulfur should be kept as low as possible. Therefore, in a non-resulfurized embodiment of this alloy, sulfur is restricted to not more than about 0.06%, better yet to not more than about 0.030%, and preferably to not more than about 0.020%.
At least about 3.7% chromium is present to benefit the hardenability provided by this alloy. To that end the alloy preferably contains at least about 4.0%, and better yet, at least about 4.25% chromium. Chromium combines with available carbon to form chromium carbides. In doing so it depletes the alloy of carbon. Such carbon depletion tends to increase the value of xcex94C such that the hardness and toughness provided by the alloy are adversely affected. Therefore, chromium is restricted to not more than about 5.0% in this alloy.
Cobalt is present in this alloy because it benefits both the room temperature hardness and the hot hardness provided by the alloy. For that purpose, the alloy contains at least about 6%, better yet, at least about 7%. and, preferably, at least about 7.5% cobalt. Too much cobalt can adversely affect the good toughness provided by this alloy. Therefore, cobalt is restricted to not more than about 12%, better yet to not more than about 11%, and preferably to not more than about 10.5% in this alloy.
This alloy contains at least about 12.0% tungsten to benefit the secondary hardness, wear resistance, and the hot hardness provided by the alloy. If the amount of tungsten is too low, the value of xcex94C becomes too negative which adversely affects the hardness and toughness provided by the alloy. Accordingly, the alloy preferably contains at least about 12.25%, and better yet, at least about 12.5% tungsten. When too much tungsten is present in the alloy, the value of xcex94C becomes too positive which adversely affects the hardness capability of the alloy. Therefore, tungsten is restricted to not more than about 13.5% in this alloy.
Vanadium contributes to the temper resistance and the secondary hardening response that are characteristic of this alloy. Vanadium combines with available carbon to form vanadium carbides which contribute to the good wear resistance provided by this alloy. The vanadium carbides also help control the grain size of the alloy during the austenitization heat treatment by pinning the grain boundaries. For these reasons, at least about 4.5% vanadium is present in this alloy. We have also found that when at least about 5.0% vanadium is present and xcex94C is maintained within the aforesaid ranges, the alloy provides unexpectedly improved toughness at the elevated hardness levels that are characteristic of the alloy. Too much vanadium adversely affects the hardness and toughness provided by this alloy. More specifically, excessive vanadium can cause brittleness in this alloy. Also, if vanadium is not properly balanced with carbon in this alloy, the hardness of the alloy will be adversely affected if there is insufficient carbon to combine with vanadium. Therefore, vanadium is restricted to not more than about 7.5%, better yet, to not more than about 7.0%, and preferably, to not more than about 6.5%.
A small amount of molybdenum may be present in this alloy in substitution for some of the tungsten. Preferably, molybdenum is restricted to not more than about 1.0% because too much causes xcex94C to become more positive, which adversely affects the high hardness provided by the alloy.
The balance of the alloy is iron except for the usual small amounts of impurities that are present in commercial grades of high speed tool steel alloys intended for similar service or use. More specifically, nickel and copper are restricted in this alloy to minimize retained austenite in the alloy after high temperature austenitizing heat treatment. Although up to 0.75% nickel or up to 0.75% Cu can be present in this alloy, when both are present, the combined amount of nickel and copper is restricted to not more than about 0.75%. Preferably, not more than about 0.50% nickel-plus-copper is present in this alloy. Up to about 0.1% magnesium and up to about 0.1% titanium can be present in this alloy. In addition, the alloy may pick up nitrogen when it is atomized with nitrogen gas. However, it is expected that no more than about 0.12%, preferably not more than about 0.08% nitrogen is present in nitrogen-atomized metal powder made from this alloy. Phosphorus is restricted to not more than about 0.030%.
This alloy can be made by any conventional process known for making high speed tool steels. Preferably, the alloy is produced by powder metallurgy techniques. For example, a heat is melted and atomized, preferably with nitrogen gas to form a metal powder. The metal powder is screened to the desired mesh size, blended, and consolidated to a substantially fully dense billet or other shape. Consolidation is carried out by any known process such as hot isostatic pressing, rapid isostatic pressing, or simultaneous compaction and reduction. The resulting compact is then subjected to further mechanical working as by press forging, rotary forging, or rolling.